Loud hHum from Subwoofer

I would check the caps on the power supply. They are rather large electrolytic capacitors that will look like they burst and/or leaking. When these die, the caps ground and the sub will make a loud buzzing sound. If you are handy with a soldering iron you can replace them yourself. Replace them with the same voltage or a little bigger and same with the size.

If the cap. says 16v 4700pF on it, for example, a 20v 5300pF or a 16v 6200mF cap. will work for this application.

How to Stop a Subwoofer Hum - HowStuffWorks

electronics.howstuffworks.com/how-to-stop-subwoofer-hum.htm
Would you like to learn how to stop your subwoofer from making a consistent low-frequent hum or buzz? Read this article to learn how to stop a subwoofer hum.

Hum usually indicates a power supply problem. If it has an internal supply you will need someone to check it's producing clean DC. Alternatively if you do open it (beware risk of shock!!) initially visually check condition of /replace any bulging electrolytic capacitors.

It sounds like you are describing an earth loop problem. I won't go into detail in this post, but try an isolation test first. disconnect everything from the reciever except the sub and try connecting say a BATTERY powered mp3 player or cd player to minimise connections to the electrical mains earth. you could also isolate things by taking your sub to a friend's house and trying it on their system.
If it is an earth loop use a process of elimination to find out which two pieces of equipment causes the earth loop. then apply filtering.
you can buy earth loop isolation transformers for about AU$26 and I've used one on my sub myself. Theyhave RCA (phono)plugs on each end and they connect inline with the signal cable to the sub amp.

as for the thumping sound it might be the speaker protection circuitry cutting off the amp to protect the subwoofer speaker from damaged. This might be triggered if the earth loop sound is causing the sub amp to overload and clip.

Failing this there might be a dried out overheated electro capacitor in the amp circuit causing the hum. Hav a chat to your local electronics person, who should be able to spot it!
Hope this puts you on the right track
Happy Hunting!

Humming noises are often caused by a faulty Electrolytic Capacitor in the power suppy section of the subwoofer.

check and make sure the ristors are not rusted

I had the same problem for many months. I asked a sound & video professional and he suggested I move the subwoofer away from any power source. I had the subwoofer next to my UPS. Once separated and moved all wiring away from the power supply (about 12") the low bass hum was practically gone.

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make sure you have the speaker connection right if you do then it is a ground problem try plugging it in a different outlet

just try leaving it unplugged for a while thats what i did and it fixed it